Recent optical semiconductor devices have higher and higher output powers, and resins for use in the optical semiconductor devices should have satisfactory thermal stability. Typically, sealants for blue/white light emitting optical semiconductors suffer from yellowing of resins caused by light and heat emitted from an optical semiconductor element. Such a sealing resin, when yellowed, absorbs light emitted from the optical semiconductor element, and this reduces the luminous intensity of light emitted from the device.
As a possible solution to improvements in thermal stability, there has been known a technique of using a composition including a monoallyl diglycidyl isocyanurate and a bisphenol-A epoxy resin (Patent Literature (PTL) 1). However, if this material is used as a sealant for a high-output blue/white light emitting optical semiconductor device, coloring of the resin proceeds by the action of light and heat emitted from the optical semiconductor element, and the colored resin absorbs light to be outputted, resulting in decreasing luminous intensity of the device with time.